Chapter 11 Observable Patterns of Inheritance

Scientist Father of inheritance: Gregor Mendel

Mendel’s Insight Into Inheritance Patterns Natural selection suggested that a population could evolve if members showed variation in heritable traits Variations that improved survival chances would be more common in each generation –in time, population would change over time or evolve

Mendel’s Experimental Approach Gregor Mendel used experiments in plant breeding and a knowledge of mathematics to form his hypothesis –He used green pea plants in his experiment This plant can self-fertlize itself True-breeding (white flower  white flower) Cross-fertilized by human manipulation of the pollen –Mendel crossed-fertilized true-breeding garden pea plants having clearly constrasting traits

Genetic terms (found on p. 179) Genes: units of information about specific traits Locus: Each gene has a location of chromosomes Homologous chromosome: diploid cells have two genes (a gene pair) for each trait Alleles are various molecular forms of gene for the same trait

Continue… True-breeding lineage occurs when offspring inherited identical alleles, generation after generation Hybrid Non-identical alleles Homozygous alleles: both alleles are the same –RR: Homozygous Dominant –rr: Homozygous Recessive

Continue… Heterozygous: One allele is dominant and other is recessive –Rr: Heterozygous Dominant Dominant: more organisms in the population with particular trait (represent with a capital letter) Recessive: very few organisms in the population with the trait (represent with a lowercase letter)

Continue… Genotype: the genetic makeup of an organisms –How many are homozygous dominant, heterozygous dominant, and homozygous recessive Phenotype: Physical appearance –Adjectives (describe the appearance) –How many are dominant and recessive?

Continue… P- Parent Generation F 1 First generation F 2 Second generation

Law of Segregation States that 2n (diploid) organisms inherit two genes per trait located on pairs of homologous chromosomes During meiosis the genes segregate from each other such that each gamete will receive only one gene per trait.

Monohybrid Crosses Monohybrid crosses have two parents that are true-breeding for contrasting forms of a trait Self-fertilization: one form of the trait disappears in the first generation offspring (F 1 ), only to show up in the second generation We know now that all members in the second generation are heterozygous

F 2 Generation A genetic cross of a F 2 generation would show up a 3:1 phenotypic ratio

Steps to a Monohybrid Cross Look up on the board to see the steps to a monohybrid cross

Testcross To support his concept of segregation, Mendel crossed F 1 plants with homozygous recessive individuals A 1:1 ratio of recessive and dominant phenotypes supported his hypothesis

Law of Independent Assortment States that during meiosis each gene of a pair tends to assort into gametes independently of other gene pairs located on nonhomologous chromosomes Mendel showed F 1 were all dominant for purple flowers and tall

Dihybrid Crosses Steps on the board

Incomplete Dominance A dominant allele cannot completely mask the expression of another Snapdragons

Codominance ABO blood types Both alleles are expressed in heterozygotes –Humans with both proteins are designated with blood type AB

Multiple Allele System Whenever more than two forms of alleles exist at a given locus –Example: Four blood types: A, B, AB and O

Multiple Effects of Single Genes Pleiotropy: The expression of alleles at one location can have effects on two or more traits

Example: Marfan Syndrome (Abe Lincoln) –Gene for codes for a variant form of fibrillin1, a protein in the extracellular matrix of connective tissues Causes weakening connective tissues throughout the body Effects: lanky skeleton, leaky heart valves, and weakened blood vessels, lens displacement

Interactions Between Gene Pairs Epistasis: One gene pair can influence other gene pairs, with their combined activities producing some effect on phenotype Example: Coat Color in Mice –Chicken Combs (Red portion on their heads)

Hair Color in Mammals In Labrador retrievers, one gene pair codes for the quantity of melanin produced while another codes for melanin deposition Another gene locus determines whether melanin will be produced at all

Comb Shape in Poultry Sometimes interactions between two pairs results in a phenotype that neither pair can produce alone Comb shape in chickens is of at least four types depending on the interactions of two gene pairs Rose Pea SingleWalnut

Regarding the Unexpected Phenotypes Tracking even a single gene through several generation may produce results that are different Camptodactyly (Immobile, bent fingers) can express itself on both or one hand because a possible gene product is missing

Polygenic Traits Traits expressed by more than one gene Eye Color (Three genes play a role of eye color)

Environmental Effects Fur on extremities of certain animals will be darker because the enzyme for melanin production will operate at cooler temperature but is sensitive to heat on the rest of the body. Hydrangea Plants: the color of floral cluster based on the acidity of the soil